Hamatulatsu (Combat)Source Inner Sea World Guide pg. 287 (Amazon), Pathfinder Campaign Setting pg. 83 (Amazon)
You have mastered a deadly fighting form inspired by the hideous attacks of the barbed devil.
Prerequisites: Improved Unarmed Strike, Weapon Focus (unarmed strike).
Benefit: Your unarmed attacks can deal either bludgeoning or piercing damage. You decide which type of damage you deal whenever you attack a foe, but you may only choose one type at a time. If you critically hit a foe with your unarmed strike while doing piercing damage, the additional pain caused by the strike causes the foe to become sickened for 1 round (or staggered for 1 round if the target is already sickened). Multiple critical hits in a round against a single foe do not increase the duration of the sickened or staggered condition.
Special: Hamatulatsu may be selected as a substitute bonus feat at 6th level by a monk even if the monk does not otherwise meet the prerequisites. This fighting style is normally only taught to women of the Sisterhood of the Golden Erinyes.Source Weapon Master's Handbook pg. 25 (Amazon)
By spending 5 stamina points, you can treat a hit with an unarmed strike as a critical hit for the purpose of determining the benefits that you gain from Hamatulatsu.